1. Field of the Invention
The present invention relates to the transmission of data within digital systems. More specifically, this invention relates to the restoration and detection of such transmitted data.
While the present invention is described herein with reference to a particular embodiment, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional embodiments within the scope thereof.
2. Description of the Related Art
The interest in local-area networks (LANs) is steadily increasing. Local area networks facilitate economical data communication between computing systems clustered in a locality. One such network used widely in the art is known as a token ring LAN.
A token ring LAN is a circular network having a plurality of stations (nodes) interconnected in a ring topology. The nodes of the system (e.g., computers, printers and other devices) are connected to a cable and assigned a unique address. Access to the network is controlled by the possession of a signal "token". The token is a packet of signals that is passed from node to node. The node that has the token has control of the network with respect to the transmission of data to other nodes and the receipt of data from same. When the node has completed a transmission, the token is released for acquisition by another node.
Errors in data transmission within the ring can generally be minimized by operating the ring in a synchronous mode. One method of synchronizing operation of the system nodes is to key all nodes to a particular master clock and extract the clock information from transmitted data with the aid of a phase-locked loop (PLL). In this regard the Manchester coding scheme is favored as allowing for relatively simple clock extraction.
In Manchester encoded data transmissions the first half of each data bit is the inverse of the last half. Accordingly, a zero crossing is present at every midbit interval irrespective of the specific data pattern. A Manchester encoded "preamble" is often transmitted together with a data pattern to enable the receiver to synchronize with and lock on to the transmitter clock signal. The receiver is thus able to distinguish individually received data bits, and to synchronize its internal processes to those of the transmitter without the aid of a separately transmitted clock signal.
As implied by the above, clock information may be recovered from a Manchester waveform simply by determining the time of each zero crossing. Hence, in certain LANs, sinusoids, derived from the bi-phase data patterns, are transmitted over the network data bus. The sinusoids are generated by passing the binary Manchester data through a low-pass filter prior to transmission on the data bus. Hence, higher-order harmonics are removed from the data waveform while the essential zero-crossing information is retained. This transmission scheme is advantageous in that transmission of the analog sinusoid may be effected without the dispersion generally accompanying digital transmissions over coaxial, twisted-pair, and other copper media.
The incidence of these sinusoidal data packets upon a receiver network is typically sensed by level detectors commonly known as "squelch" circuits. Squelch circuits are used to enable or disable a receiver circuit. Squelch circuits detect signal energy in excess of a predetermined voltage threshold. The detection threshold is generally selected to be less than the anticipated energy level of the sinusoidal transmissions. In addition, a squelch reset threshold is chosen to be commensurate with the expected DC component (DC offset) of the incident sinusoidal waveform. Unfortunately, variation in the DC component of the sinusoidal data packet (i.e. common mode noise) may cause conventional squelch circuits to produce errant detection signals.
Squelch circuits typically operate in conjunction with "slicer" data restoration circuits disposed to extract zero-crossing information from the transmitted sinusoid. Rather than endeavoring to directly ascertain the location of each zero-crossing, slicer circuits are designed to detect signal energy at threshold levels during zero-crossings of either polarity. If the thresholds are chosen to be relatively close to zero, then the phase distortion introduced into the reconstructed data waveform will be minimal. However, selecting detection thresholds relatively close to zero increases the probability that noise energy carried by the incident sinusoid will cause the slicer circuit to register false zero crossings. Hence, in conventional slicer data restoration circuits the detection thresholds are chosen by balancing the competing demands of noise immunity and reduced phase distortion. Again, common mode variation in the average signal energy introduces a phase error into the recovered waveform and exacerbates the aforementioned difficulties associated with common mode noise.
Both slice and squelch circuits have been conventionally realized using Schmidt triggers. In a Schmidt trigger, a closed loop network is formed by linking the output and non-inverting inputs of a comparator with a resistive feedback path. Unfortunately, the single non-differential input arrangement of a Schmidt trigger results in susceptibility to the adverse effects of common mode signal variation described above. Moreover, the comparators included within Schmidt triggers are often ill-suited to drive the resistive feedback loop at high frequency.
Accordingly, a need in the art exists for level detection and data restoration arrangements having decreased sensitivity to common mode signal variation and improved high-frequency response.